1. Field of the Invention
The present invention relates to a wire electric discharge machining apparatus, and more specifically to a mechanism for supplying electric power to a wire electrode of the wire electric discharge machining apparatus.
2. Description of Related Art
A wire electric discharge machining apparatus applies voltage between a wire electrode and a workpiece, and generates an electric discharge between the wire electrode and the workpiece to thereby carry out the machining. Therefore, the wire electric discharge machining apparatus has a power supply element brought into contact with the wire electrode to supply power thereto. However, the wire electrode runs during the machining, so that the power supply element is abraded at a contact point with the wire electrode, thus forming a groove or the like if the contact position is fixed. This makes the contact conduct insufficient, and the desired voltage cannot be provided to between the wire electrode and the workpiece.
In order to solve this problem, a well-known wire electric discharge machining apparatus is configured to rotate a power supply element so that the contact position of a wire electrode on a power supply element is not fixed and to move the power supply element straightforward in a direction perpendicular to the running direction of the wire electrode to vary the contact position of the wire electrode on the power supply element, thereby lengthening the life of the power supply element (see JP 8-19920A, JP 2001-232520A and JP 2552851B, for example).
There is another well-known wire electric discharge machining apparatus, which is provided with a power supply element between the first and second electrode guides, and drives the second electrode guide to vary the contact position of the power supply element on a wire electrode, thereby extending the life of the power supply element (see JP 2671663B, for example).
In the process of changing the contact position of the wire electrode on the power supply element by moving the power supply element, the application of voltage to the power supply element generates a secondary abrasion due to an electrical phenomenon, such as a small discharge, on sliding surfaces of the power supply element and a supporting portion. As a measure against the secondary abrasion, it is possible to conceive of the idea that the supporting portion for supporting the power supply element can be formed into an insulating structure. This measure, however, would cost more accordingly.
On the other hand, in the process of changing the contact position of the wire electrode on the power supply element by moving the electrode guide, the wire electrode is supported by the electrode guide at a wide angle since the electrode guide is moved. Thus, the wire electrode runs while generating a large abrasion against the electrode guide, and the electrode guide is therefore abraded soon. As a result, it is required to frequently exchange the electrode guide for a new one. Furthermore, the mechanism for changing the position of the electrode guide becomes complicated and thus costly.